This invention relates to strain gages in general and more particularly to a strain gage accelerometer configuration for detecting movements of an inertia member in a force transmitting environment.
Basically, an accelerometer measures acceleration or, more particularly, the force that is exerted when a body possessing inertia is accelerated. The inertia tends to resist the acceleration. The force according to Newton's law or the reaction is equal to the product of the mass and the acceleration.
Many different types of accelerometers exist in the prior art and basically can operate to measure acceleration by performing measurement on a mass which may be coupled to a spring assembly, or some other device, which due to its resilency or movement, will compress or stretch for movement of the mass.
Various common types of accelerometers employ different types of transducing arrangements as well as different configurations.
Certain devices called angular strain gage types use two strain gages which form part of a bridge circuit and are mounted on a sensing beam. One can derive a sinusoidal or variable frequency output which is related to velocity or acceleration.
In other systems as the damped spring mass system, a seismic mass is the sensor. The mass is retained by a spring and damper in a housing. The mass can therefore move relative to the housing and when the housing is accelerated, the resultant displacement of the mass is measured.
Certain other transducers employ piezoelectric devices which use a crystal and housing in conjunction with a mass and may employ a spring and so on as an elastic member. The crystal produces an output proportional to the movement of the mass.
Another type of device is a stretched wire accelerometer where a mass is supported by two wires within a housing. If the housing is accelerated along the direction of the wire axis, tension of one wire increases while that of the other wire decreases. Strain pick off is in each wire and their sum is proportional to acceleration.
The piezoresistive transducer has also been employed for measuring strain in transducer configurations as indicated above. It has been known in the prior art that the piezoresistor is traditionally more sensitive than, for example, metal wire or foil type strain gages.
Certain prior art patents as U.S. Pat. No. 3,444,499 entitled STRAIN GAGE issued on May 13, 1969 to Endevco shows piezoresistive elements which are shaped into an hour glass configuration and thus, possess a narrow neck. This element is then mounted across a slot in a beam or mass. Due to the fact that the neck is narrow, the strain is concentrated and thus amplified. Further, the material used in the piezoresistive elements is a semiconductor material as silicon, which possesses a greater resistance per unit of strain than typical prior art devices.
Other patents as U.S. Pat. No. 3,363,471 entitled ACCELEROMETER show different slot configurations on seismic masses employing specially shaped piezoresistive elements having associated contacts therewith and positioned across a slot in a housing to measure displacement of the respective walls of the mass to provide outputs proportional to acceleration.
Other configurations of such piezoresistive strain gages can be had by referring to U.S. Pat. No. 3,351,880 entitled PIEZORESISTIVE TRANSDUCER issued on Nov. 7, 1967.
Generally, one can characterize the performance of an accelerometer by two quantities which are its output per acceleration (g) and its natural frequency. It is desireable to maximize both quantities in any given design; but because the greater the mass of the seismic structure, the greater the output per g but the lower the natural frequency.
In order to eliminate the effect of mass, one frequently refers to a quantity designated as the figure of merit of the accelerometer system. The figure of merit is given by the product of the output per g times the natural frequency squared. Since the output per g is directly proportional to the mass and the natural frequency is proportional to the square root of the system's stiffness divided by the mass, the figure of merit will be independent of the mass.
In any event, due to the increasing demands of present technology, it is desireable to fabricate an accelerometer with high figures of merit, excellent thermal characteristics and enhanced ruggedness.
It is further desireable to employ such a device having an improved frequency response to enable one to therefore measure relatively high frequencies, small magnitude accelerations.
The problem in regard to certain prior art techniques resides in the fact that the strain gage is mounted or positioned directly across a narrow gap formed in a seismic mass. While this technique enabled one to obtain a high figure of merit, it presented a number of problems in attempting to bridge the gap directly with the strain sensitive elements. When the strain gage is placed directly across the gap, it acts as the only spring restoring member in the seismic system and its deflection is the maximum deflection that the system can accommodate. The strain in the sensor is given by .DELTA.l divided by l; where .DELTA.l is the deflection of the sensor and l its effective length. Thus, for a given deflection, a narrow gap and a shorter effective length will result in a high figure of merit. However, the strain gage which is bonded at both ends has poor thermal characteristics since the center portion or strain responsive portion is not in contact with the mass material.
Furthermore, due to the narrow neck of the gage, one would experience breakage or fracture of the piezoresistive material for high shock loads.
In conjunction with these problems, one still could not obtain a significant improvement in the resonant frequency of the device due to the fact that the length and cross section of the gage, which has to bridge the slot or gap, are determined by criteria other than the most desireable spring constant.
Accordingly, to prevent buckling of the sensor, the sensor had to be as short and thick as possible. These characteristics are contrary to obtaining a sufficient resistance to make proper measurements as this resistance specifies that the sensor be as long and thin as possible. Thus, the electrical properties of the sensor are diametrically opposed to its mechanical properties.
Also as indicated, one experiences a problem in attempting to match the thermal expansion coefficient of the semiconductor sensor to the thermal expansion coefficient of the seismic mass. Since the sensor is usually fabricated from silicon, which has a low expansion coefficient, after the semiconductor is bonded across a gap, it will be placed in precompression and thus, in fact, be more susceptible to buckling.
It is therefore an object of this invention to provide an improved transducer arrangement for use in a slotted beam transducer or accelerometer device; which device possesses a high natural frequency, a high figure of merit and good thermal characteristics while exhibiting reliable operation.